This invention relates to a piston rod position detector for detecting the position of a piston rod connected to a piston slidably mounted in a cylinder so as to protrude from the cylinder. It also relates to an autotensioner and a belt transmission device carrying such a piston rod position detector and to a belt tension adjusting device with a tension pulley position detector.
A cylinder unit is used to convert hydraulic force to mechanical force and transmit the mechanical force to a device through a piston rod. It is also used to measure a moving distance.
A cylinder unit of the former type is shown in FIG. 12 and is generally called an “autotensioner” in the art and is used to apply tension to the timing belt in a belt transmission device.
FIG. 12 shows such a belt transmission device used to drive automotive engine camshafts. It comprises a pulley P1 mounted on an engine crankshaft 1, pulleys P2 mounted on camshafts 2, a pulley P3 mounted on an oil pump drive shaft 3, and a timing belt 4 trained around these pulleys. The camshafts 2 and the drive shaft 3 are driven as the crankshaft 1 rotates. The belt transmission device further includes a belt tension adjustor comprising a tension pulley 9 rotatably mounted on a pulley arm 6 which is pivotable about a shaft 5, and an autotensioner 7 having a pushrod 8 protruding from a cylinder and biased by a spring, not shown, to press the tension pulley 9 against the belt 4 to absorb any change in belt tension, thereby keeping the belt tension constant.
The tension in the belt changes as it stretches gradually with time or due to changes in the distance between pulleys due to thermal expansion during operation of the engine. The autotensioner absorbs any such change in the belt tension by advancing and retracting the pushrod. For example, when the belt stretches and slackens, the pushrod 8 advances to absorb slack of the belt.
Such autotensioners are disclosed e.g. in Japanese patent No. 1891868 and Japanese patent publication 7-117130.
If the belt transmission system has no such autotensioner, it is usually necessary to exchange the timing belt each time the vehicle has traveled 100 thousand kilometers. The autotensioner suppresses flapping of the belt and thus prolongs its life, so that the belt guarantee period can be extended until the vehicle travel distance far exceeds 100 thousand kilometers.
But since the pushrod stroke is limited, if the pushrod has advanced beyond its stroke limit, the autotensioner loses its ability to keep constant the belt tension. The belt thus tends to get slack and begins to flap. This may cause flapping or “jump of pulley teeth”. Also, flapping of the belt shortens the life of the belt. Also, the timing belt stretches, so that the pushrod protrudes too much and the timing belts fails to function properly. One may therefore think of providing a means for detecting the fact that the pushrod has advanced to the stroke limit or to a position near the stroke limit. A specific such means would be a detector including a sensor (coil) and a nonmagnetic ring fitted on the push rod. When the edge of the nonmagnetic ring reaches the center of the sensor, magnetic permeability changes. The detector thus detects a specific position of the pushrod.
But with this arrangement, only one specific position of the pushrod is detectable. Thus, it is desired to detect the position continuously or a plurality of different positions of the pushrod.
As with an autotensioner, in spite of the fact that it is known that as the travel distance of a vehicle increases, a timing belt stretches unnoticed and the position of the pushrod of the autotensioner changes, the change in the amount of protrusion of the pushrod is not actually detected. During the process of increase in the amount of protrusion of the pushrod, if alarms such as “caution” at half the elongation at expected breakage, “warning” at 80% elongation, and “broken” at the breakage point are given according to the elongation of the belt, measures regarding an exchange period of the belt can be taken. But no such measures are taken now. Also, there are various kinds of devices for which it is desirable to take similar measures like autotensioners.
An object of the invention is to provide a piston rod position detector capable of detecting the position of a piston rod continuously or at a plurality of different positions and to provide an autotensioner provided with such a detector.
FIG. 33 shows a belt transmission device with a belt tension adjustor which is another type of autotensioner.
A timing belt 4 is trained between a pulley P1 of a crankshaft 1, pulleys P2 of camshafts 2, and a pulley P3 of a drive shaft 3 for an oil pump. The autotensioner shown is one disclosed in Japanese patent publication 8-338488 and is a compact type autotensioner in which a hydraulic damper is housed in the periphery of a tension pulley 9 for compactness.
For the autotensioner, the tension pulley 9 is provided so as to be pressed against the timing belt 4 so as to be pivotable about a shaft 5, and keeps the tension of the timing belt constant by the tension adjusting force of a spring and the hydraulic damper. The tension pulley 9 pivots counterclockwise to relax tension when the tension is excessive, and pivots clockwise when the tension is decreasing to adjust tension. As the use period extends, the entire length of the timing belt tends to stretch due to secular change. Thus, as the operating period and use period extend, the tension pulley 9 of the autotensioner pivots clockwise.
The pivoting angle of the tension pulley in the autotensioner is limited, so that as operation exceeding the pivot limit point continues, it becomes impossible to keep the tension of the timing belt constant. The tension of the timing belt decreases, so that due to flapping of the timing belt, the belt tends to deteriorate. This will ultimately cause breakage of the belt.
But no detecting mechanism is provided for detecting the pivoting angle of the tension pulley to detect whether or not it is operated beyond the pivot limit point. Thus, it is impossible to know if the tension of the timing belt is kept properly, and to properly judge the period for exchanging the timing belt, and thus to know beforehand even if there is an abnormality in the autotensioner.
Another object of this invention is to provide a belt tension adjusting device provided with a means for detecting the position of the tension pulley and indicating the exchange period of the timing belt or abnormality of the autotensioner.